The field of the invention is medical imaging, and particularly, the production of real-time anatomic images used to guide the performance of medical procedures.
High resolution medical images which accurately depict detailed anatomical features can be produced using a number of imaging modalities. Such images may be produced, for example, using electron beam CT systems, ultrasound systems or MRI systems commercially available from a number of manufacturers. To reconstruct high resolution and high SNR images, however, considerable data must be acquired. Such data acquisition requires time ranging from seconds to many minutes depending on the imaging modality used and the particular subject of the acquisition. For example, when imaging the human heart both cardiac and respiratory gating may be used during the data acquisition with the result that image data may only be acquired during short intervals during each respiratory and cardiac cycle. A high resolution 3D image of the heart may require many seconds or minutes to acquire using an MRI system or an x-ray CT system. Such high resolution images are commonly used for diagnosing disease, but they cannot be acquired and reconstructed at a high enough frame rate for use during medical procedures.
Medical images are produced at high frame rates for use during medical procedures. For example, x-ray fluoroscopy is commonly used by physicians to guide catheters in the placement of balloons and stents during angioplasty procedures. MR fluoroscopy is a method used to produce real-time images for monitoring medical procedures, and real-time ultrasound images are used in intravascular procedures such as those disclosed, for example, in U.S. Pat. Nos. 5,325,860 and 5,345,940. Such real-time imaging methods produce images at a frame rate of 10 to 30 frames per second to provide a relatively continuous, flicker-free view of the procedure being monitored.
While real-time medical images can thus be produced with a number of well-known imaging modalities, such images do not depict anatomy in great detail or with great clarity. They do depict medical instruments with enough clarity, however, that the instruments can be seen and guided to carry out some medical procedures.
There are medical procedures which require both real-time imaging to guide the operation and high resolution, high SNR depictions of the anatomical structures being operated upon. One such procedure is guided ablative therapy for cardiac arrhythmias. Ablative therapy employs an electrode which is placed on or near the heart wall and energized to create lesions of specific dimensions in specific anatomic locations. These lesions disrupt or change the sequence in which the heart wall muscle activates during an arrhythmia, and if done accurately, will prevent the arrhythmia from occurring.
As disclosed in U.S. Pat. Nos. 5,325,860 and 5,345,940, both the ablation electrode and an ultrasonic transducer may be delivered to the interior of the heart intravascularly by a catheter. The ablation electrode is guided into position using the real-time ultrasonic images and the ablation process is monitored using the real-time ultrasonic images. While this method and apparatus has enabled certain types of arrhythmias to be treated successfully, improved accuracy in the placement of the ablation electrode is required before the method can be used to treat many other cardiac arrhythmias. To do this, an accurate and clear image of the underlying anatomy must be provided in real-time to the physician so that the ablation electrode can be placed in precisely the right location.
The present invention is a method and means for providing clear, high resolution medical images in real-time, and more particularly, providing such medical images to assist physicians in the performance of medical procedures. The invention includes: acquiring image data of the subject anatomy and reconstructing an image which is a high resolution model of the subject anatomy; performing a medical procedure in which the subject anatomy is imaged in real-time by acquiring low resolution images at a high frame rate; registering the high resolution model of the subject anatomy with each acquired low resolution image; and displaying to the physician in real-time images of the registered high resolution model of the anatomy. The high resolution model may be a 2D or 3D image of static anatomy, or it may be a 4D model in which the fourth dimension depicts changes in the anatomy as a function of time, cardiac phase, respiratory phase, or the like. The creation of this model is performed using a high resolution imaging modality and it may be done prior to performing the medical procedure. The registration of the high resolution model is performed in real-time and includes a 2D or 3D spatial orientation as well as a registration in time or phase when the model depicts changing anatomy.
Another aspect of the present invention is the merging of real-time images of physiological data with an anatomical model during a medical intervention procedure. The physiological data may be, for example, electrophysiological data, EEG data or thermal data. If a medical device such as an endocardial electrode array is used, for example, the device itself may not be displayed. Instead, the location of each electrode in the array may be precisely located on the registered high resolution anatomic model and an electrical activation map is produced from the acquired electrophysiological data and overlayed on, or merged with the high resolution image. In a preferred embodiment the overlay is done by modulating the color of the pixels to indicate the activation timing of the imaged tissues, however other display methods may also be used.
Yet another aspect of the present invention is to accurately display the location of a medical device as a medical procedure is performed. The low resolution images clearly depict medical devices located within their field of view. Devices can thus be properly located in the displayed images by simply overlaying them on the registered high resolution model. In the case of an ablation catheter, for example, this overlay is simply the depiction of the device itself.
Yet another aspect of the present invention is the production of real-time medical images having a wide field of view. The field of view of many real-time imaging systems is very limited, or it may depict a single slice or a single projection view. It requires great skill and practice on the part of the physician to relate such limited views to the subject anatomy and properly understand the image. By registering the high resolution model of the anatomy to the limited real-time image, however, a much larger and more understandable field of view depicted by the model can be displayed. Indeed, the registered model may depict the subject anatomy in 3D and the images displayed to the physician can create a virtual reality environment during the medical intervention procedure.